1. Field of the Invention
The present invention relates to a photo timer for controlling n irradiation apparatus so that the dose of radiation does not exceed predetermined value when a subject is irradiated with the radiation during radiography, or the like. The present invention also relates o a radiographic apparatus which includes the photo timer.
2. Description of the Related Art
Nowadays, various kinds of X-radiographic apparatuses have been proposed and are used in the field of X-radiography for medical diagnoses or the like. In the X-radiography, a solid state detector (which includes a semiconductor as its main part) is used as an X-ray image detection means. The solid state detector detects X-rays which have been transmitted through a subject and obtains an image signal representing an X-ray image related to the subject.
Further, various types of solid state detectors which may be used in the X-radiographic apparatuses have been proposed. For example, if the solid state detectors are classified according to an electric charge generation process for converting X-rays into electric charges, there are solid state detectors of a photo conversion type, solid state detectors of a direct conversion type, and the like. In the solid state detector of the photo conversion type, signal electric charges are obtained at a photo-conductive layer by detecting fluorescence emitted from phosphors which have been irradiated with X-rays. Then, the obtained signal electric charges are temporarily stored in a storage unit. The stored electric charges are converted into an image signal (electric signal), and the image signal is output. Meanwhile, in the solid state detector of the direct conversion type, when the photo-conductive layer is irradiated with X-rays, the signal electric charges are generated in the photo-conductive layer. The generated signal electric charges are collected at electric charge collection electrodes. The collected signal electric charges are temporarily stored in a storage unit. Then, the stored electric charges are converted into an electric signal, and the electric signal is output. The main parts of the solid state detector of this type are the photo-conductive layer and the electric charge collection electrodes.
If the solid state detectors are classified according to an electric charge readout process for reading out the electric charges stored in the solid state detectors to the outside of the solid state detectors, there are solid state detectors of a photo readout type, solid state detectors of a TFT readout type, and the like. In the solid state detector of the photo readout type, the solid state detector is irradiated with readout light (electromagnetic wave for readout), and the electric charges are read out from the solid state detector. In the solid state detector of the TFT readout type, as disclosed in U.S. Pat. No. 6,828,539, TFT's (thin-film transistors) are sequentially driven along scan lines, and the electric charges are read out from the solid state detector.
Further, solid state detectors of an improved direct conversion type have also been proposed in U.S. Pat. No. 6,268,614 and the like. The solid state detectors of the improved direct conversion type have both the characteristics of the direct conversion type and those of the photo readout type. In the solid state detectors of the improved direct conversion type, a photo-conductive layer for recording, an electric charge transfer layer, and a photo-conductive layer for readout are stacked together in this order. The photo-conductive layer for recording is a layer which becomes photo-conductive when it receives recording light (X-rays, fluorescence generated by irradiation of X-rays, or the like). The electric charge transfer layer is a layer which acts substantially as an insulator for an electric charge which has the same polarity as a latent image electric charge, and which acts substantially as a conductor for a transfer electric charge which has a polarity opposite to the latent image electric charge. The photo-conductive layer for readout becomes photo-conductive when it is irradiated with electromagnetic waves for readout. In the solid state detectors of the improved direct conversion type, signal electric charges (latent image electric charges) which carry image information are stored at the interface (storage unit) between the photo-conductive layer for recording and the electric charge transfer layer. Further, electrodes (a first conductive layer and a second conductive layer) are stacked at both sides of the three layers. The main parts of the solid state detector of this type are the photo-conductive layer for recording, the electric charge transfer layer, and the photo-conductive layer for readout.
Besides the apparatuses using the solid state detectors as described above, various kinds of X-ray image detection means such as imaging plates and films are used in medical radiography. In all of these cases, a photo timer is generally used during X-radiography. The photo timer is used to obtain a high-quality image and to prevent excessive irradiation of a patient during radiography. The photo timer is used to detect the dose of X-rays, with which the X-ray image detection means has been irradiated. Then, the detected X-ray dose is used to control the dose of X-rays, with which the patient is irradiated. Further, an X-radiographic apparatus of a cassette type, in which a photo timer as described above is incorporated, is proposed in Japanese Unexamined Patent Publication No. 2000-010220, for example.
However, when the photo timer as described above is used, it is required that an X-ray irradiation apparatus and the photo timer are connected to each other by a cable, and that the photo timer is placed close to the X-ray image detection means during photography. However, this configuration is not convenient for users. Further, in the X-radiographic apparatus of the cassette type, in which the photo timer is incorporated, as disclosed in Japanese Unexamined Patent Publication No. 2000-010220, it is also required that the incorporated photo timer and the X-ray irradiation apparatus are connected to each other by a cable. However, this configuration is not convenient for the users. Further, if they are connected by the cable, the flexibility in radiography is limited.